Field of the Invention
This invention relates to microwave frequency filters. More specifically, this invention relates to a microwave frequency cavity filter whose bandwidth can be precisely fine-tuned with a minimum of effort, expense, and service interruptions.
Discussion of the Related Art
The rapid growth in cellular telephony and wireless communications has created enormous demand for bandwidth in the microwave radio frequency spectrum. As wireless technologies that depend on the microwave spectrum have become more popular, the microwave portion of the radio spectrum has become more crowded. Unused microwave frequencies are occupied by wireless service providers as soon as they become available, forcing wireless communication firms operating in the same location to provide their services on adjacent frequencies, without the benefit of any xe2x80x9cemptyxe2x80x9d bandwidth between them. Because of this congestion, wireless providers need a way to isolate the transmission and reception of their frequencies from neighboring frequencies that are used for other services or by other providers.
To accomplish this frequency isolation, resonator filters have been developed. These filters are built to permit only the frequencies in a certain range to pass through. This frequency range is called the pass band, and the frequencies inside this range are called bandpass frequencies. The frequencies outside of the pass band fall into the stop bands, and are blocked by the filter.
While a number of resonator filter designs have been developed, one of the most common filters for use in microwave communications is the cavity filter. This type of filter consists of a number of resonators placed inside physically adjacent hollow metal cavities, thereby forming cavity resonators. By inductively coupling two or more adjacent resonators, the bandpass frequencies of these resonators are combined, forming a resonator filter with a bandwidth encompassing a range of frequencies.
But in order to properly block the undesired frequencies in the stop band of the filter, some physically adjacent resonators in the filter are capacitively cross-coupled, which effectively cancels out certain frequencies in the filter. Capacitive cross-coupling attenuates the slope of the frequency response curve of the filter between the edge of the pass band and the edge of the stop band, allowing the filter to more precisely match the desired pass band without also erroneously passing frequencies outside of the pass band that may be used for other signals or which may be owned by other service providers. In essence, adjusting the capacitive cross-coupling within the filter fine tunes the isolation of the filter.
In this regard, capacitive cross-coupling and inductive coupling have the opposite effect on the signals passed between adjacent resonators. For this reason, conventional cavity filters do not employ both capacitive cross-coupling and inductive coupling between a given pair of resonators.
In conventional cavity filters, the inductive coupling between adjacent resonators is accomplished by placing a gap in the wall separating the two cavities. The size of the gap determines the amount of coupling. A common method of providing the capacitive cross-coupling in these conventional filters is to extend a metal bar across the wall separating two electrically non-adjacent resonators. The length of the bar determines the capacitive cross-coupling. In order to precisely select the frequency cutoff of the filter between the pass band and the stop band, the cross-coupling bar must have very precise physical dimensions.
Furthermore, in order to fine tune the filter for tolerance purposes, the physical length of the bar must be changed, either by means of a fine tuning screw at one end of the bar, or more commonly by replacing the bar with another one of different length.
But adjustment of the capacitive cross-coupling by either means is cumbersome and impractical. First, conventional cavity filters used for microwave signals are quite large and are made entirely of metal with covers or lids made of lead that cover the resonator cavities as well as the cross-coupling bars. Replacing or adjusting the cross-coupling bar requires physically removing this lead cover, which is difficult and labor intensive.
Furthermore, manufacturing the cross-coupling bars to the precise physical dimensions and tolerances required in conventional filters makes them expensive, which adds further to the overall cost of the filter.
Given these problems with conventional filters as well as the increased need for precise tuning of filter bandwidth at low cost, what is needed is a cavity filter that can be manufactured at a reduced cost but whose bandwidth can be very precisely tuned and adjusted with a minimum of effort and without interruption of service.
The invention is directed to a cavity filter. According to a first aspect of the invention, the resonator comprises a filter housing having at least two cavities separated by a cavity wall; a filter cover for covering said filter housing; and a plurality of resonators respectively disposed in said cavities, wherein at least two of the resonators are coupled to each other by both an inductive coupler and a capacitive cross-coupler.
Specifically, the capacitive cross-coupler includes a bar that extends from the cavity wall into each of the cavities and the inductive coupler is an opening in the cavity wall between the cavities. The inductive coupler also includes an adjustable fine tuner comprising a screw threaded through either the filter cover or the filter housing, such that the screw extends into the opening in the cavity wall.
The invention is also directed to a method of fine tuning the slope of the frequency response curve of the cavity filter described above by attenuating the capacitive cross-coupling effect indirectly by adjusting the fine tuner of the inductive coupler. Specifically, the fine tuner is adjusted from the exterior of the filter by turning the screw further into the opening in the cavity wall, thereby increasing the inductance of the inductive coupler, reducing the capacitance between the two resonators. Similarly, by unscrewing the screw, it is retracted from the opening, reducing the inductance of the coupler and increasing the capacitance between the two resonators.